[llvm] [CI][Github] Enable CIR CI build and test (PR #147430)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

@andykaylor / @joker-eph Are you able to come to the infra area team meeting this week? I'd like to ensure we have community consensus on the additional compute resource, latency costs, and additional maintenance/error burden and I think that's the most reasonable forum to do it.

Although there really shouldn't be any concerns if we're just enabling CIR checks on CIR changes, probably not a bad idea to start the discussion now.

https://github.com/llvm/llvm-project/pull/147430